The present invention relates to a Transponder System. In particular the invention relates to remote identification and telemetry systems wherein an electronic sub-system called an interrogator including a transmitter and receiver extracts by electromagnetic means useful information from an electronically coded label, sometimes known as an electronic tag, attached to a person or object about which information is desired.
A block diagram of a type of system to which the invention relates is shown in accompanying FIGS. 1A and 1B. This system uses the principle of radio frequency backscatter in which an interrogator containing a transmitter radiates an electromagnetic signal via an interrogator antenna toward an electronic label containing a label antenna. As a consequence of electromagnetic coupling between the label antenna and the interrogator antenna, the label receives a portion of the radiated signal. In the label, the impedance of a termination connected to the label antenna may be varied in time under control of a modulator containing an electronic code generator, with the result that electromagnetic energy received by the label antenna is both partially reflected by the termination and re-radiated by the label antenna in a time varying manner. As a further result of the electromagnetic coupling between the label antenna and the interrogator antenna, a portion of the time varying radio frequency signal scattered by the label may enter a receiver included within the interrogator wherein it may be separated from the transmitted signal, amplified. decoded in a decoder and presented in digital or analog form to other systems which may make use of the information obtained by the interrogation.
In the label, operations of the code generator and variable termination may be controlled in time by an oscillator. In a passive label the circuits within the modulator may receive power for their operation from a rectifier circuit, while in an active tag they may receive at least part of their operating power from a separate energy source, for example a battery. When the rectifier circuit is present, the effects of the time varying termination can be provided by the rectifier circuit itself, in which variation of dc load directly presented to the rectifier by the modulator may cause a corresponding variation in the load presented by the rectifier to the label antenna.
In the interrogator, there may generally be provided a controller which may perform various calculation and control functions and may also participate in the decoding process.
Practical applications of such labelling and interrogation systems lie in identification of, and or monitoring condition of, such things as persons, goods, shipping containers, pets and livestock. In the latter case, the tags are sometimes attached to the exterior of the animal, and are sometimes implanted beneath the skin.
In the practical design of such systems various problems arise. In a passive tag, one problem is that of obtaining from the rectifier circuit sufficient voltage for operation of the modulator circuits within the label at a transmitter power level sufficiently low to meet licensing, economic or health requirements.
A further problem, applying principally to passive labels, derives from significant variation in strength of the electromagnetic field normally experienced by a label as it moves throughout an electromagnetically scanned area. For the type of oscillator circuit which it is practicable to use in the most economically manufactured integrated electronic labels, this variation of field strength and hence rectified dc voltage causes significant consequential variation in frequency of the on-chip oscillator. Since almost all forms of reply signal encoding employ time as an important parameter, such variation can cause difficulties or errors in decoding, or may require that a significant portion of the information in the reply be devoted to the task of indication of frequency of the on-chip oscillator. In normally used forms of reply signal decoding, circuits in the receiver must firstly detect the frequency of the modulation signal used in constructing a reply, and after this frequency is detected, determine the reply signal information content. If the label is, by virtue of its motion, only exposed to the interrogation signal for a limited period, a portion of which is used in frequency determination, the amount of useful information which is recoverable from the label is limited by the problem discussed.
Although this problem can be alleviated to a degree by use of a voltage regulator within the label, the label rectifier system must then supply for satisfactory operation the sum of the voltage drop required across the regulator and the minimum operating voltage for the oscillator circuit, with the result that an effective reduction in label interrogation range occurs.
A further problem, which affects labels being read at all but very short distances, is that the reply signal reaching the interrogator antenna is very weak in relation to the interrogation signal and yet occupies a very similar region of the frequency spectrum. The receiver is therefore very susceptible to any component of the phase noise spectrum of the transmitter which may reach it. The usual technique for separation of the reply from the interrogation signal involves firstly use a circulator or directional coupler to effect gross separation of signals flowing in the forward and reverse directions in the interrogator antenna transmission line, and then to use linear signal mixing to move the reply signal to a lower frequency band, wherein amplification and filtering may be conveniently applied.
In the gross separation discussed above, the effectiveness depends upon the degree of isolation which may be achieved by the coupler or circulator, and also on the extent to which the interrogator antenna is matched to its transmission line, as imperfections in antenna matching have the same effect as imperfect isolation in the coupler or circulator. If insufficient matching is achieved, the mixer which effects frequency conversion of the reply signal will become overloaded by the imperfectly removed interrogation signal, and will not function properly, and in addition excessive noise, generated within the transmitter as near sidebands of the interrogation signal, will reach the receiver. The result is that extremely accurate interrogator antenna matching is required for successful system operation at large interrogation ranges. Such extremely accurate matching cannot normally be sustained in the face of typically encountered changes over time in the antenna environment, including those introduced by movement of labelled and unlabelled objects within the field of the antenna, so the hoped for interrogation range is not achieved, even though the reply signal level be well above both thermal noise and the level of other stray reflected signals in its pass band.
A further problem is present in homodyne detection systems normally used for detection of the reply signal. In such systems a low power version of the transmitted signal is used in the interrogator receiver as the local oscillator for the linear mixer which transforms the reply signal to the previously said lower frequency band. Because the reply is derived by direct modulation within the tag of the transmitted signal, there will be phase coherence between the reply and the local oscillator derived from the transmitter. The result will be that the mixer will produce zero base band output whenever the reply is in quadrature with the local oscillator, as indeed it will be whenever the difference in propagation times from the transmitter to mixer, via the tag and via the local oscillator sampling path, is a quarter of a period of the transmitter oscillation. Because in a radio frequency backscatter system the phase of the reply varies considerably as the label moves within the scanned area, this effect will produce positions of the label for which no reply is observed.
The usual way in which this problem is met is to divide the reply signal into two portions, each being fed to a separate mixer, the two mixers having local oscillator signals derived from the transmitter but with a ninety degree relative phase shift, the mixer outputs being again combined to a single data stream after one of them has been provided with a further ninety degree phase shift at the base band frequency. Although this remedy does produce a form of base band signal for all positions of a label with linear amplitude or angle modulation of the interrogation signal, the base band signal so obtained contains distortion which may prejudice successful decoding unless the spectrum of the signal is relatively narrow band, i.e. takes the form of a modulation signal of limited bandwidth superimposed on a significantly higher frequency sub-carrier. Since the frequency of this sub-carrier can be no greater than the frequency of the oscillator within the tag from which it is derived, and since considerations of economical use of power received by the tag set a practical upper limit to this frequency, the need in this kind of detection system to use a narrow band modulation sets an unwelcome limit to the rate at which data may be transmitted within the reply, and a consequential limit to the amount of data which may be recovered from a label which by virtue of its motion is within the interrogation field for only a short period of time. As will be explained in more detail later, there are in addition, in the replies from practical labels, departures from the idealised forms of modulation of the interrogation signal for which this style of interrogator receiver is not suited, in that for some positions of such practical labels no reply might be observed.